1. Field of the invention
The present invention relates to an acoustically insulating electrical box in particular but not exclusively of the type designed to receive a conventional, domestic A.C. switch or outlet. The invention also relates to a method of acoustically insulating an electrical box of the above type.
2. Brief description of the prior art
U.S. Pat. No. 4,296,870 issued on Oct. 27, 1981 to BALKWILL et AL proposes a cover for a metallic electrical box mounted in an opening cut in a building wall or ceiling board. This cover, made of plastic material, extends around all of the electrical box except the open face thereof and is provided with a slanted flange which contacts and seals against the back surface of the wall or ceiling board. The function of the cover of BALKWILL et AL is to prevent passage of air and moisture through and around the electrical box. It is not mentioned in U.S. Pat. No. 4,296,870 that the plastic cover acoustically insulates the electrical box.
It is very questionable as to whether the slanted flange of BALKWILL et AL produces sufficient pressure on the back of the board to result into a joint impervious to aerial acoustic waves. In any case, it is very important that a constant, sufficient pressure be applied by the flange on the back of the board around all of the open face of the box. This accordingly results in a tedious, time consuming and therefore costly installation of the covers of BALKWILL et AL.
BALKWILL et AL further propose to attach the electrical box on the stud structure by means of nails drived through the box cover. These nails cancel any acoustic insulation effect produced by the slanted flange applied on the back surface of the wall or ceiling board as they cause an opening in the cover allowing passage of acoustic waves. Moreover, the nails establish an acoustic short circuit between the electrical box and the stud structure allowing transmission of acoustic vibrations from the box to the stud structure and of course from the stud structure to the box.
When the metallic, electrical box is nailed on a vertical stud through the cover of BALKWILL et AL, any twisting in the stud causes lack of parallelism between the cover flange and the board, variation in the pressure applied by the flange on the back surface of the board and accordingly a deficiency in the joint between these flange and board.
BALKWILL et AL further suggest mounting of a plurality of metallic, electrical boxes through a pair of metallic straps again nailed to the stud structure. A plastic cover for such a box assembly is provided with slots to allow passage of the straps. Again, the joint between the metallic straps and the plastic material of the cover is not tight enough to stop aerial acoustic waves. Again, the mounting straps, made of metal, cause an acoustic short circuit between the metal of the electrical boxes and the stud structure whereby acoustic vibrations are transmitted through these straps from the stud structure to the electrical boxes, and vice versa.